linux-kernel - Re: [PATCH] mm: memcontrol: fix blocking rstat function called from atomic cgroup1 thresholding code

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Date:   Mon, 26 Jul 2021 15:16:06 +0000
From:   Rik van Riel <riel@...com>
To:     "hannes@...xchg.org" <hannes@...xchg.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
CC:     Kernel Team <Kernel-team@...com>,
        "dan.carpenter@...cle.com" <dan.carpenter@...cle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>
Subject: Re: [PATCH] mm: memcontrol: fix blocking rstat function called from
 atomic cgroup1 thresholding code

On Mon, 2021-07-26 at 11:00 -0400, Johannes Weiner wrote:
> 
> __mem_cgroup_threshold() indeed holds the rcu lock. In addition, the
> thresholding code is invoked during stat changes, and those contexts
> have irqs disabled as well. If the lock breaking occurs inside the
> flush function, it will result in a sleep from an atomic context.
> 
> Use the irsafe flushing variant in mem_cgroup_usage() to fix this

While this fix is necessary, in the long term I think we may
want some sort of redesign here, to make sure the irq safe
version does not spin long times trying to get the statistics
off some other CPU.

I have seen a number of soft (IIRC) lockups deep inside the
bowels of cgroup_rstat_flush_irqsafe, with the function taking
multiple seconds to complete.

Reviewed-by: Rik van Riel <riel@...riel.com>